Yes, A&I have covers for the helicals, but I dont think they have any in stock as I just tryed yesturday to order some from my local dealership. You can see them on loewen's site.
I do the same thing setting the seives. Eyeball for the crop, then fine tune accordingly. Have you plugged the tailing at all? If not, I wonder if you can close the bottom just a tickle. ?
I'm looking forward to getting to use my new Loewen helicals. I've been using the Gorden helicals, but they have no wires. On approx. a 13.5" wide concave, the helicals have an extra 2" of bar surface, plus the allmost true 90 degree to material flow orientation of the bars, should really make things happen. Looking at the angle,a guy might think a stock straight bar concave would act similar to a transport vane than a concave which is to retard material.
I'd about bet that if you were to do a quick kill on the machine, without any cover plates, you'd find that the broken wheat heads are falling through the concave too soon. IE: some heads are even broken before they reach the concaves. Then as soon as the head hits the concave, it is forced through the wires before it can make another revolution with the straw material to rub against it and thresh the grain out.
In grass seed I tryed the no cover plates a few years back to soo if I could gain some capacity. What happened was the seed heads did'nt get enough rubbing before it was sent through the concave. We had a lot of what is called "doubles". That is in a way similar to the white caps and head tips you see. Doubles are 2 or more seeds still bound together by plant material. As soon as I installed the cover plates, the doubles went away. Once the doubles were gone, it made it easier to make other adjustments to clear up other issues that might slow me down.
Here's a pic of the gorden helicals. They have no wires, but the bars are closer together and smaller metal. I like them. They do a good jobe. I am going to the loewen helicals with the wires to se if i can make the air-jet chaffer work in grass. The air-jet is a great concept and I'd like to see it work. However the problem has allways been the amount of straw spearing the screen get. I'm hoping that the wires and the other changes I made this year will help that spearing problem. We'll see.